The present invention is related to a class-D amplifier.
Class-D amplifiers perform power amplifications by pulse-width-modulating (PWM) input signals, and are utilized so as to perform power amplifications of audio signals. As related class-D amplifiers, there is such a class-D amplifier which is arranged by a pulse width modulating unit for pulse-width-modulating analog input signals, a low-pass filter positioned at a post stage of the pulse width modulating unit, and a damper circuit positioned at a post stage of the low-pass filter. A speaker, or the like, which constitutes a load is connected parallel to this damper circuit.
FIG. 6 shows an example of a conventional class-D amplifier (self-excited type). A pulse width modulating unit provided in this conventional class-D amplifier is arranged by a resistor R1, an operational amplifier 11, a capacitor C1, a comparator 12, a switch driving circuit 13, a switch circuit 14, and a resistor R2. The resistor R1 constitutes an input terminal of an analog input signal. Both the operational amplifier 11 and the capacitor C1 constitute an integrator which integrates the analog input signal. The comparator 12 compares an output of the integrator with a predetermined reference value. The switch driving circuit 13 outputs a pulse signal corresponding to an output signal of the comparator 12 as a BTL (Bridged Tied Load) type signal. The switch circuit 13 is driven by an output of the switch driving circuit 13. The resistor R2 constitutes a negative feedback circuit which feeds back the output of the switch circuit 14 to the input side of the integrator in a negative feedback manner. A low-pass filter constructed of both a coil “L” and a capacitor “C2” is connected at a post stage of this pulse width modulating circuit. A damper circuit constituted by series-connecting the capacitor C6 to the resistor R4 is connected at a post stage of this low-pass filter. In other words, one end of this damper circuit is connected to the output terminal of the above-described low-pass filter, and the other end of the damper circuit is connected to the earth. A load 16 constructed of a speaker, or the like is connected parallel to the damper circuit. The above-explained damper circuit damps a peak of an output which is produced at a resonant frequency of the low-pass filter (LC circuit) when the class-D amplifier is brought into either no load condition or a light load condition.
As a conventional power amplifier, there is such a power amplifier equipped with a class-D power amplifying circuit, a peak voltage detecting circuit for detecting a peak voltage of the output voltages of the class-D power amplifying circuit, and a drive voltage control circuit arranged at a prestage of the class-D power amplifying circuit (for example, refer to FIG. 1 of patent publication 1). This peak voltage detecting circuit detects the peak value of the output voltages of the class-D power amplifying circuit, and controls the drive voltage control circuit based upon the detected peak value, so that the output peak of the class-D power amplifying circuit is suppressed.
As a conventional protection circuit of a motor driving circuit, such a protection circuit is provided. That is, while a plurality of transistors (drive elements) are connected by way of a bridged connecting manner so as to constitute a motor driving circuit, a diode is connected to one end of this transistor in order to protect this transistor (for instance, refer to FIG. 1 of patent publication 2). patent publication 1;
Japanese Laid-open Patent Application No. Hei-3-55905 patent publication 2:
Japanese Patent Publication No. Hei-6-59011
However, in the conventional class-D amplifier (self-excited type) shown in FIG. 6, in order to damp the output peak produced at the resonant frequency of the low-pass filter (LC circuit) in the case that the conventional class-D amplifier is brought into the light load condition, the above-explained damper circuit (constructed of capacitor C6 and resistor R4) is required. A loss current may flow through the resistor 4 functioning as the structure elements of this damper circuit even in the case that the conventional class-D amplifier is operated under the normal load condition. As a consequence, in the above-described conventional class-D amplifier, the damper circuit is necessarily required, which impedes the improvement in the efficiency of this class-D amplifier.
The conventional power amplifier described in the above-described patent publication 1 owns such a problem that in order to suppress the output peak of the class-D power amplifier, the complex circuit such as the peak voltage detecting circuit and the drive voltage control circuit other than the class-D amplifier is required.
Further, in the conventional protection circuit of the motor driving circuit described in the above-mentioned patent publication 2, although the transistors functioning as the drive elements are protected, the high voltage may be produced across the load of the motor driving circuit. Both a coil and a capacitor which are directly connected to the motor (load) cannot be protected, and thus, there are some possibilities that these coil and capacitor are electrically destroyed.